Crystal Structure
- Practical Electron Microscopy and Database -
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A classical definition of a crystal is: A crystal is a substance in which the constituent atoms, molecules, or ions are packed in a regularly ordered, repeating three-dimensional pattern. [1]

Table 3264 summarizes the some basic differences between crystals, glasses, and liquids.

Table 3264. Some basic differences between crystals, glasses, and liquids.

Substances
Basic differences
Crystals
Long-range ordering
Glasses
Short-range ordering
Liquids
No ordering to short-range ordering

A lattice point is known as a motif or basis. We can obtain a crystal structure by combining the lattice with the motif (i.e., crystal structure = lattice + motif). Figure 3264 shows a periodic pattern consisting of a two-dimensional (2-D) net and a motif. The motif is arranged symmetrically and is repeated at each point of the 2-D net to create the periodic pattern, and thus the lattice structure is also symmetric.

 (a) (b) (c)
Figure 3264. (a) A 2-D net, (b) A motif, and (c) Formed periodic pattern (2-D lattice structure).

Crystalline specimens have to be tilted in a goniometer in TEM in order to:
i) Observe lattice fringes and crystal structures.
ii) Determine the crystal orientation.
iii) Observe diffraction contrast of lattice defects with certain Bragg reflections or known orientation.
iv) Determine the Burgers vector of lattice defects.

[1] International Union of Crystallography [prior to 1992].

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